1. Technical Field
The present invention relates to a polyvinyl chloride resin composition for use as an insulator of insulated electric wires for use in automotive wire harnesses, electronic appliances, etc. In particular, the invention relates to a ultrathin-wall low-voltage electric wire for automobile, which is obtained by thinly coating a conductor with the polyvinyl chloride resin composition as an insulator for automotive low-voltage electric wire.
2. Background Art
As a result of trends toward electronization in vehicle-mounted devices, etc., the number of electric/electronic wiring circuits within the motor vehicle has increased remarkably and the spaces within the motor vehicle which are occupied thereby have increased, resulting in an increase in the overall weight of the motor vehicle due to the automotive electric wires. Such increases in the weight of insulated electric wires have resulted in a decrease in the fuel economy of motor vehicles. Being lightweight is desirable from the standpoint of an improvement in fuel economy, and the increase in the use amount of insulated electric wires is contrary to a reduction in the weight of insulated electric wires. However, from the standpoint of mounting microcomputers in a motor vehicle and driving the vehicle safely and comfortably, it is impossible to avoid increases in the amount of automotive electric wires used. Under these circumstances, investigations on a weight reduction in automotive electric wires are being made recently.
The weight of automotive electric wires to be used can be reduced by reducing the diameters of automotive electric wires or by reducing the thickness of the insulators of electric wires.
In general, polyvinyl chloride resin compositions consisting mainly of polyvinyl chloride are being used as the insulators of insulated electric wires for use in wire harnesses for motor vehicles and in electronic appliances, etc.
A plasticizer has been incorporated into these polyvinyl chloride resin compositions. Increasing the incorporation amount of the plasticizer enhances the flexibility of the polyvinyl chloride resin composition and can improve the cold resistance thereof. However, the increased plasticizer amount results in decreases in mechanical property, e.g., abrasion resistance.
In recent years, with the trend toward weight reduction in motor vehicles and size reduction and precision enhancement in electronic appliances, the wire harnesses and insulated electric wires to be used therein have come to be required to be reduced in weight. For attaining the weight reduction, the electric wires are required to be reduced in diameter and the insulator with which a conductor wire is coated is required to have a reduced thickness. Reductions in the diameter of electric wires and in coating thickness are prone to result in a further decrease in abrasion resistance. Although it is possible to improve the abrasion resistance by reducing the amount of the plasticizer to be incorporated and thereby increasing the hardness of the material, the mere reduction in plasticizer amount results in a decrease in the cold resistance of the polyvinyl chloride resin composition, making the electric wire unable to retain low-temperature characteristics.
Because of this, the insulators of these insulated electric wires generally are constituted of a resin composition consisting mainly of polyvinyl chloride, and currently have a thickness down to about 0.35 mm since this resin composition in a thin film form has poor mechanical strength, in particular, poor abrasion resistance.
A technique in which such a resin composition including polyvinyl chloride as the main component is applied in a thickness of 0.08-0.2 mm is described, for example, in JP-A-10-241462.
JP-A-10-241462 discloses a thin-wall insulated electric wire wherein an insulator including polyvinyl chloride as the main component and having a Shore D hardness of 72 or higher has been formed by extrusion molding in a thickness of 0.08-0.2 mm, and wherein the Shore D hardness of 72 or higher is obtained by regulating the amount of an octyl ester of trimellitic acid to be incorporated.
A technique in which a resin composition including polyvinyl chloride as the main component is applied in a thickness of 0.16-0.10 mm is described, for example, in JP-A-8-7652.
JP-A-8-7652 discloses an automotive thin-wall low-voltage electric wire obtained by extrusion coating with a vinyl chloride resin composition in a thickness of 0.16-0.10 mm, the vinyl chloride resin composition being configured by incorporating 5-25 parts by weight of a plasticizer, 4-8 parts by weight of a stabilizer, 0-10 parts by weight of a filler, and 1-5 parts by weight of an acrylic processing aid into 100 parts by weight of the sum of a graft PVC copolymer with which an acrylic rubber has been graft-polymerized and PVC, the amounts of the graft PVC copolymer and the PVC being in the ranges of 100-20 parts by weight and 0-80 parts by weight, respectively.
Namely, either a graft PVC copolymer alone with which an acrylic rubber has been graft-polymerized or a combination of the graft PVC copolymer and PVC is compounded in a amount of 100 parts by weight with a plasticizer, a stabilizer, a fuller, and an acrylic processing aid to configure the resin composition.